Summary

Cisco IOS Software contains a vulnerability that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on a targeted device.

The vulnerability is due to improper handling of malformed packets. An unauthenticated, remote attacker could exploit this vulnerability by sending malicious network packets to the targeted device. If successful, the attacker could cause the device to become unresponsive, resulting in a DoS condition.

Cisco confirmed the vulnerability in a security advisory and released updated software.

Indicators of Compromise

Cisco has published a list of affected Cisco IOS Software releases in the security advisory. The Vendor Announcements section of this alert contains a link to the advisory.

Technical Information

The vulnerability is due to improper handling of IP protocol 91 packets on devices that are running Cisco IOS Software using Data-Link Switching (DLSw).

An unauthenticated, remote attacker could exploit this vulnerability by sending malicious network packets to the device. When processed, the packets could cause the device to fail and reload, resulting in a DoS condition.

Analysis

Only devices that are running Cisco IOS Software that uses the DLSw feature in promiscuous mode are affected by this vulnerability. Devices with only statically configured DLSw peers are not vulnerable to exploitation.

Cisco indicates through the CVSS score that functional exploit code exists; however, the code is not known to be publicly available.

Safeguards

Administrators are advised to apply the appropriate updates.

Administrators may consider using IP-based access control lists (ACLs) to allow only trusted systems to access the affected systems.

Administrators are advised to monitor affected systems.

The Cisco Applied Intelligence team has created the following companion document to guide administrators in identifying and mitigating attempts to exploit this vulnerability prior to applying updated software: cisco-amb-20110928-dlsw

Vendor Announcements

Fixed Software

Cisco customers with active contracts can obtain updates through the Software Center at the following link: Cisco. Cisco customers without contracts can obtain upgrades by contacting the Cisco Technical Assistance Center at 1-800-553-2447 or 1-408-526-7209 or via e-mail at tac@cisco.com.

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